Electrical transmission systems often require well-controlled impedances. Basically, a transmitting circuit has a signal source which generates a voltage signal which passes through a source impedance to a terminal. The terminal is connected to a second receiving circuit having its own impedance acting as a load impedance to the first circuit. If the system is duplex, i.e., signals will also be transmitted from the second circuit to the first circuit and the roles of the two circuits are reversed, then the load impedance is further connected to a second signal source. In this case, the impedance of the first circuit acts as a source impedance during the transmission of signals from the first circuit to the second circuit and as a termination impedance upon the reception of signals from the second circuit.
In the design of such circuits, various requirements may be imposed upon the impedance of the circuit. It may be desirable that the impedance be well-controlled and accurate or perhaps different at different signal frequencies. A complex impedance having capacitive or inductive characteristics may be required.
Moreover, for the overall design of these circuits, the ease of implementation into integrated circuit form with a minimum of separate components would be a favorable feature, due to the resulting compactness in physical size and good reliability.
Furthermore, if the circuit is a differential one, the suppression of common mode signals would be a likely requirement.
All of these features above are particularly desirable in a telephone system. Such a system has communication carrier channels comprising a pair of unidirectional transmission paths. By an interface circuit located in a telephone office at each end of the channel, the unidirectional paths, one carrying an incoming signal and the other carrying an outgoing signal, are connected to a balanced two-wire, bidirectional transmission path. This bidirectional path forms the subscriber loop to which is connected the subscriber's terminal equipment, such as a telephone hand set.
In the past, the interface circuit has been implemented by so-called transformer hybrid circuits. Due to the need of matching impedances for signal balancing at both the carrier side and the subscriber side by means of transformers, costs of such transformers are high.
Additionally, transformer impedance matching has been with nominal values of impedances for the unidirectional carrier paths and the bidirectional subscriber paths, which has made the performance of the transformer hybrid circuit less than optimum. Another disadvantage of the transformer hybrid has been that the subscriber loop must be supplied with a large DC current to power the subscriber terminal equipment. Since AC signals, such as those of audio frequency, are superimposed upon the DC current for communication to and from the terminal equipment, the transformer in the hybrid circuit must be made to handle the DC current and be responsive to the AC signal. This results in additional costs and size for the hybrid circuits.
Various designs to avoid transformers in hybrid circuits have been made. One such design having current mirror subcircuits has resulted in U.S. Pat. No. 4,004,109, issued Jan. 18, 1977, by F. S. Boxall. However, one disadvantage of the Boxall design is that not only must some of the resistance elements in the circuits be matched, but also the value of these elements must be precisely set, for its proper operation. These requirements raise the manufacturing costs of such a circuit. Furthermore, the Boxall design does not provide for complex impedances for an optimal matching to the transmission paths which nearly always have significant complex impedances.